The plane Truth.
A few things;
Before you can plane wood flat….You first have to understand the nature of wood.
Wood is flexible. if the wood you are using is thick and strong it may not flex when you push the plane across it, it is thin it may flex a lot. Wood can also move when you plane it due to internal stress, so wood needs to be stable to start with, or you are going to be chasing your tail. If you are doing a large table top, then it is especially important to make sure that it is properly supported without creating any distortion to it.
Once you determine that then you proceed to shimming the wood to support it from flexing if need be, or at least from rocking.
Next is to understand the nature of flat.
Two points; A straight line is the shortest distance between two points.
Three points; Flat is the plane formed by three non-colinear points. Connect those three points with straight lines and you have a flat plane.
Five points; Are used to define a flat rectangle. The four corners and the centre.
Now take your winding sticks….. throw them away…. and get yourself a good straight edge!
With your straightedge check, across the grain, lengthways with the grain and across the diagonals.
The diagonals will tell you if your board is twisted….. where and by precisely how much.
You may find that using shims on a large surface like a table top may help; Put a penny or a dime on each of the four corners and one in the centre, lay your straightedge across the diagonals, if your board is flat, the straightedge will touch all three points across both diagonals.
If it rocks, the middle is high.
If it doesn’t touch the center; the corners are high.
Plane down to the lowest point, even amounts from each corner, if they are high.
Constantly check both diagonals! To end up flat, the straightedge must touch all three points across both diagonals. The five points define the flat plane and everything must be faired in to those points, check with your straightedge across and lengthways and plane down all of the high spots. If you have spots that are lower then your five reference spots, then you must lower you five reference spots.
After some practice you will learn to read it and do it quicker.
Once you wrap your head around what you are attempting to do and get some practice, it will become second nature to you and you will remove the minimum amount of wood from the right place to get to your goal.
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